1. Field of the Invention
The present invention relates to a light-emitting device, a lighting device, a display device, and a semiconductor device. In particular, the present invention relates to a light-emitting device having an organic EL element and a manufacturing method thereof.
2. Description of the Related Art
An organic EL element which is a light-emitting element in which a layer containing a light-emitting organic compound is provided between a pair of electrodes (e.g., an upper electrode and a lower electrode) is known. The layer containing a light-emitting organic compound has an extremely small thickness, which is greater than or equal to several tens of nanometers and less than several hundreds of nanometers, and breaks down easily.
Although the organic EL element emits light by causing current to flow therethrough, Joule heat is caused depending on current flowing through the element. In some cases, the Joule heat damages the layer containing a light-emitting organic compound and reduces the reliability of the organic EL element.
Further, a lighting device including organic EL elements, in which, in general, a plurality of organic EL elements which are connected in series are driven by a constant current, entirely emits no light if there is a short circuit in one of the organic EL elements. The short circuit is probably caused due to dusts, an unevenness of a lower electrode, or the like. A short circuit may occur even in a place where a short circuit has not caused yet in an initial state while an organic EL element is driven to convert current into light. Its mechanism will be described below.
(1) In some places, bright light is emitted because a large amount of current flows through part of the organic EL element due to a dust or an unevenness of the lower electrode which is slight enough not to cause a short circuit. Such a place is referred to as a luminescent spot.
(2) Since a larger amount of current flows through the luminescent spot than other places, heat generated in the luminescent spot due to Joule heat is higher than that in other places. Therefore, the temperature of the luminescent spot gradually rises if heat is insufficiently diffused throughout a light-emitting device from the luminescent spot.
(3) The electric resistance of the organic EL element gets lower as the temperature rises; accordingly, a much larger amount of current flows through the luminescent spot if the electric resistance of the luminescent spot is decreased due to generation of heat.
When the temperature of the layer containing a light-emitting organic compound in a luminescent spot rises to a temperature that cannot be withstood due to repetition of the above (2) and (3), the layer containing a light-emitting organic compound is broken down and the pair of electrodes is short-circuited.
In the lighting device including organic EL elements, a technique of dissipating heat is extremely important to cut off such a vicious cycle. Particularly in the case of a bottom-emission lighting device in which light is extracted to a substrate side on which an organic EL element is provided, it is important to efficiently dissipate heat generated by the organic EL element to a wall or a ceiling which easily diffuses heat through a counter substrate because the counter substrate is provided in contact with the wall or the ceiling.
Thus, a first light-emitting device, a second light-emitting device, and a third light-emitting device are known which efficiently dissipate heat generated by an organic EL element so that deterioration of characteristics of the organic EL element due to the generation of heat can be arrested.
The first light-emitting device includes an element substrate over which an organic EL element is formed, a counter substrate being adhered to the element substrate and forming a sealed space with the element substrate, and a thermal conductor being provided in the sealed space and conducting heat of the organic EL element to the counter substrate, and an inert liquid is used for the thermal conductor (see Patent Document 1).
In the above first light-emitting device, a short circuit is sometimes caused because a dust or the like attached to a cathode is crashed by the thermal conductor in sealing the thermal conductor between the element substrate and the counter substrate which are attached to each other using a drop-filling device. This is because, in a general drop-filling device, a pressure is applied to the element substrate and the counter substrate at their attachment so that the thermal conductor for filling is spread out uniformly.
The following measures can be considered not to apply a pressure at the attachment: a space for the thermal conductor is provided on the counter substrate side and sealing is performed with the space filled with a less amount of the thermal conductor than the capacity of the space. Accordingly, the pressure applied to the cathode at the sealing can be almost eliminated.
However, with the above measures, a region filled with a gas having low thermal conductance is formed widely in a space between the counter substrate and the thermal conductor; therefore, thermal diffusivity is drastically decreased at the center or the like of the counter substrate. In the case where the thermal conductor is a liquid, this occurs when a light-emitting device is attached to a ceiling or the like so that the element substrate faces downward. Even in the case where the thermal conductor is a solid, the thermal conductor might be solidified so that the region filled with a gas having low thermal conductance is formed widely in the space between the counter substrate and the thermal conductor. When the counter substrate is made to face downward and solidified, a region filled with a gas is formed widely in a space between the cathode and the thermal conductor.
The second light-emitting device includes, as a thermal conductor, an inert liquid between a sealing member and an organic EL element, and an uneven structure portion, which makes a heat dissipation area larger than when a surface of an element substrate is flat, is provided on the surface side of the element substrate, which is opposite to a light-emitting layer side of a cathode (see Patent Document 2).
The third light-emitting device has a structure in which heat generated by an organic EL element is dissipated to a projected heat dissipation portion which is provided in contact with the organic EL element (see Patent Document 3). However, when there is a material having low thermal conductance (e.g., a gas such as air) between the heat dissipation portion and the organic EL element or between the heat dissipation portion and a sealing substrate, heat generated by the organic EL element is not dissipated efficiently. Thus, a structure in which the heat dissipation portion is provided so as to be in contact with the organic EL element and is pressed to the sealing substrate to be in close contact with each other is known (see Patent Document 3).
However, when a foreign substance unintentionally is mixed into a space between the projected heat dissipation portion and the organic EL element or between the heat dissipation portion and the sealing substrate, the pressure applied to press the heat dissipation portion to the organic EL element is concentrated in a portion of the organic EL element where the foreign substance exists. As a result, electrodes in a pair might be in contact with each other and are short-circuited. Alternatively, the thickness of a layer containing a light-emitting organic compound gets locally smaller, which sometimes causes abnormal heat generation to such a thin portion due to a large amount of current flow. When locally heated, the layer containing a light-emitting organic compound is broken down due to the heat and thus the pair of electrodes is short-circuited in some cases. When the pair of electrodes of the organic EL element is short-circuited, current concentrates on the short-circuited portion and damages the organic EL element. Further, fire or the like might be caused.
The concentration of pressure can be probably prevented in such a manner that a space between the above-described projected heat dissipation portion and organic EL element is filled with a thermal conductor having fluidity so that the thermal conductor is deformed in accordance with the shape of the foreign substance.
Note that in the case where the thermal conductor has fluidity, a bubble that is mixed when the space is filled with the thermal conductor can move freely. Thus, in some cases, the bubble can enter a space between the heat dissipation portion and the organic EL element and accordingly heat conduction to the heat dissipation portion from the organic EL element is obstructed.
It is natural that consideration not to allow mixture of a foreign substance be taken in a manufacturing process of a light-emitting device including an organic EL element; however, it is difficult to eliminate the mixture of a foreign substance. It is also difficult to fill the space with a thermal conductor having fluidity with a bubble prevented from being mixed and without being overflowed.